Cell Biology. Stephen R. Bolsover

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1 A LOOK AT CELLS AND TISSUES

With very few exceptions, all living things are either a single cell or an assembly of cells. This chapter will begin to describe what a cell is, and further chapters will say much more. However, to begin with, we can briefly describe a cell as an aqueous (watery) droplet enclosed by a lipid (fatty) membrane. Cells are, with a few notable exceptions, small (Figure 1.1), with dimensions measured in micrometers (μm, 1 μm = 1/1000 mm). They are more or less self‐sufficient: a single cell taken from a human being can survive for many days in a dish of nutrient broth, and many human cells can grow and divide in such an environment. In 1838 the botanist Matthias Schleiden and the zoologist Theodor Schwann formally proposed that all living organisms are composed of cells. Their “cell theory,” which nowadays seems so obvious, was a milestone in the development of modern biology. Nevertheless, general acceptance took many years, in large part because the plasma membrane (Figure 1.2), the membrane surrounding the cell that divides the living inside from the nonliving extracellular medium, is too thin to be seen using a light microscope. Microorganisms such as bacteria, yeast, and protozoa exist as single cells. In contrast, the adult human is made up of about 30 trillion cells (1 trillion = 10¹²), which are mostly organized into collectives called tissues.

ONLY TWO TYPES OF CELL

Superficially at least, cells exhibit a staggering diversity. Some have defined, geometric shapes; others have flexible boundaries; some lead a solitary existence; others live in communities; some swim, some crawl, and some are sedentary. Given these differences, it is perhaps surprising that there are only two types of cell (Figure 1.2). Prokaryotic (Greek for “before nucleus”) cells have very little visible internal organization so that, for instance, the genetic material, stored in the molecule deoxyribonucleic acid (DNA), is free within the cell. These cells are especially small, the vast majority being 1–2 μm in length. The prokaryotes are made up of two broad groups of organisms, the bacteria and the archaea (Figure 1.3). The archaea were originally thought to be an unusual group of bacteria but we now know that they are a distinct group of prokaryotes with an independent evolutionary history. The cells of all other organisms, from yeasts to plants to worms to humans, are eukaryotic (Greek for “with a nucleus”). These are generally larger (5–100 μm, although some eukaryotic cells are large enough to be seen with the naked eye; Figure 1.1) and structurally more complex. Eukaryotic cells contain a variety of specialized structures known collectively as organelles, embedded within a viscous substance called cytosol. Their DNA is held within the largest organelle, the nucleus. The structure and function of organelles will be described in detail in subsequent chapters. Table 1.1 summarizes the differences between prokaryotic and eukaryotic cells.